tig: fix tig-completion's dependency on __git_complete

tig recently updated it's bash-completion making it depend on __git-complete from git.
Becase __git-complete is not automatically sourced tig bash completion fails.
Also this PR makes tig completion load on-demand.

(cherry picked from commit f57da8e)

pkgs/applications/version-management/git-and-tools/tig/default.nix

@@ -31,10 +31,13 @@ stdenv.mkDerivation rec {
     make install
     make install-doc
 
+    # fixes tig-completion __git-complete dependency
+    sed -i '1s;^;source ${git}/share/bash-completion/completions/git\n;' contrib/tig-completion.bash
+
     substituteInPlace contrib/tig-completion.zsh \
-      --replace 'e=$(dirname ''${funcsourcetrace[1]%:*})/tig-completion.bash' "e=$out/etc/bash_completion.d/tig-completion.bash"
+      --replace 'e=$(dirname ''${funcsourcetrace[1]%:*})/tig-completion.bash' "e=$out/share/bash-completion/completions/tig"
 
-    install -D contrib/tig-completion.bash $out/etc/bash_completion.d/tig-completion.bash
+    install -D contrib/tig-completion.bash $out/share/bash-completion/completions/tig
     install -D contrib/tig-completion.zsh $out/share/zsh/site-functions/_tig
     cp contrib/vim.tigrc $out/etc/